I was just looking through the stuff on MAHQ, and I noticed something that has me confused. I always assumed they numbered the Gundams Production number as they were built. However, the Destiny Gundam seems to disprove that idea. Destiny has a production number of ZGMF-X42S, which is before both Impulse (ZGMF-X56S) and Gaia (ZGMF-X88S).

Does anyone have any idea how they chose the numbers? I know with the EAF Gundams in SEED that the different hundreds were because of different kinds of purposes (Dual Buster and Strike in the 100 Series for example) but the ZAFT numbers don't seem to quite follow that, at least for the Gundams.